What will cause excessive condensation in the AHU and duct system of an AC system?

Presently system is saturated with water, which has leaked into duct insulation, and stands frozen due to cold weather conditions.

Lennox AHU AC system in attic installation. AC unit only. Separate electric heat.

2 Local installation contractors have suggested heated air from living space is entering duct system and condensing. However the amount of water present makes this answer unlikely in my opinion.
Also, homeowner indicates that filters are wet and moldy at changing.
Homeowner indicates AC system is extremely efficient, i.e. cools house very fast.

Could system be 'overcharged' to cause freezing of coil thus producing excessive condensation in ducts...